In a fiber optic gyroscope, Sagnac phase for rotation rate sensing is determined by measuring the intensity of interfering lightwaves traveling through an identical optical path of a fiber loop in opposite directions (e.g., the clockwise (CW) and counterclockwise (CCW) directions). Such an identical optical path involves a reciprocal path from both spatial and polarization-mode points of view. The correct Sagnac phase is measured only when the CW and CCW travelling lightwaves travel in the same polarization state while within the same sections of loop fibers. The so-called polarization errors occur if such lightwaves travel in different polarization modes while within the same section of loop fibers.
Conventional single-mode optical fibers, either polarization maintaining (PM) or non-polarization maintaining, normally support two polarization modes. Due to cross-couplings at fiber splices and birefringence disturbance in the fiber, there is always energy exchange between the two polarization modes. Interference of polarization cross-coupled lightwaves with the primary lightwaves or with other cross-coupled lightwaves introduces error signals that do not carry the correct phase information for rotation sensing, and thus must be avoided and reduced.
From the standpoint of reducing polarization error, PM fiber sensing coil is preferable due to smaller polarization-mode cross-couplings in the fiber. However, even PM fibers are susceptible to non-zero cross-couplings due to micro bending and non-uniform stress built into the fiber during the manufacturing process. The strength of these cross-couplings is typically characterized by an h-parameter. PM fiber coil with large value of h-parameter may result in larger polarization errors that degrade the gyroscope bias performance.
In a depolarized gyroscope, non-polarization maintaining single mode (SM) fiber is used for cost reduction and/or improved resistance to radiation. There are ways to design the optical circuit so that the polarization errors are reduced to a relatively low level. Still, most of these methods require high polarization extinction ratio (PER), ε, of the integrated optical circuit (IOC) or of a polarizer (in case an IOC is not used) because all of the polarization errors are proportional to either ε or higher orders of same. However, in many cases, the PER of an IOC or of a single polarizer is not high enough, and the polarization error may not be small enough for certain applications of the gyroscope.
Referring to FIG. 1, apparatus 1 is a typical prior-art interferometric fiber optic gyroscope that includes a broadband light source 11, a directional coupler 12, a photodetector 14, an integrated optic circuit (IOC) 16, a sensing loop 10, and fiber sections 15, 210 and 220 that connect the IOC to the coupler and the sensing loop. The sensing loop 10 may include a polarization-maintaining (PM) fiber coil or non-PM single-mode (SM) fiber coil. The IOC 16 includes a Y-shape waveguide 40, which splits the input lightwaves into substantially equal parts at junction 17, a polarizer 18, and electrodes 19 for phase modulation. In one typical configuration, the IOC waveguide 40 may be the polarizing element instead of containing the polarizer 18. The fiber sections 15, 210 and 220 may be Lyot-type depolarizers, each including two PM fiber sections with their birefringent axes oriented 45° with respect to each other. In these prior art apparatuses, the only polarizing (PZ) element is often the IOC 16. They may not have high enough PER to reduce polarizations errors below required level.